Aurornis is an extinct genus of avialan theropod dinosaurs and, perhaps, one of the very first. It contains a single species, Aurornis xui, described based on a fossil found in the Tiaojishan Formation of Liaoning, China, in rocks dated to the late Jurassic period (Oxfordian stage), about 160 million years ago. Scientists in magazine, say a feathered, chicken-sized creature known as Aurornis xui, unearthed recently in northeastern China, challenges the pivotal position of Archaeopteryx — long regarded as the oldest bird.

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Archaeopteryx is a genus of early bird that is transitional between feathered dinosaurs and modern birds and long considered the first of its kind. However, older avialans have since been identified, including Anchiornis, Xiaotingia, and Aurornis.

"Our analyses indicate it as the most primitive bird {Xui} known," said co-author Andrea Cau, a vertebrate paleontologist at the Museo Geologico Giovanni Capellini in Italy. "It was a small feathered dinosaur that lived in what is now China about 160 million years ago. It looked like a ground bird, but with a long tail, clawed hands and toothed jaws."

There is now a clearer lineage leading from non-avian dinosaurs to birds, starting with the clade of dinosaurs called Maniraptora ("hand snatchers").

"The maniraptoran theropods are the animals most similar to Archaeopteryx and early birds, and thus are the best candidate as avian ancestors," Cau said. "In particular, we found that the earliest birds were very similar to the earliest troodontids, a kind of maniraptorans."

Theropoda is both a suborder of bipedal dinosaurs, and a clade consisting of that suborder and its descendants (including modern birds). Dinosaurs belonging to the suborder Theropoda were primarily carnivorous, although a number of theropod groups evolved herbivory, omnivory, and insectivory. Among the features linking theropod dinosaurs to birds are a furcula (wishbone), air-filled bones, brooding of the eggs, and (in some cases) feathers.

The Los Angeles Times wrote:

"The authors argue that Aurornis represents the earliest known bird, but other scientists say it could be part of a group of bird-like dinosaurs that were developing feathers and bird-like features but never quite got off the ground, evolutionarily speaking."

"You're looking at an animal that is either a very primitive bird or something very closely related to birds,' said [Luis] Chiappe, a veterbrate paleontologist at the National History Museum of Los Angeles who is not involved in the Nature study. 'I tend to think that it's not a bird, but that it's one of those true very ancestors of bird."